Doppler radar velocity measurement horn

ABSTRACT

Doppler radar apparatus for use in a velocity measuring system for farm tractors and the like. The apparatus includes a dual mode conical horn having a flare angle substantially in excess of 12.5°, and a dielectric lens formed of a glass filled polymer. The doppler output signal provided by the RF transceiver associated with the dual mode horn is high pass filtered to remove low frequency signals therefrom. The horn is mounted in such a way that it is isolated from mechanical vibrations which would induce doppler signal frequencies in excess of the cutoff frequency of the high pass filter. Moreover, the horn assembly (which is aluminum) and a steel housing assembly are coupled together in such a way that thermal expansion and contraction will not loosen the friction fit between the two assemblies. The two assemblies are held together by a single, large diameter annular locking ring.

BACKGROUND AND FIELD OF THE INVENTION

The present invention relates to doppler radar apparatus for use inmeasuring the velocity of vehicles such as a farm tractors and the like.

Various operations performed by agricultural equipment must becontrolled as a function of the speed at which the equipment istravelling. For example, contemporary agriculture often requires thedistribution of liquid or solid herbicides, pesticides, fertilizers,etc. over the area in which crops are or will be planted. If thequantity of liquid or solid applied per unit area is inexact orincorrect, it can decrease the effectiveness of the material beingdistributed or increase the cost of distribution. A similar distributioncontrol problem arises during planting, in which the spacing betweenadjacent seeds also affects the cost of the planting and the maximumcrop yield. The accuracy of the distribution of seed and other materialsper unit area depends upon the accuracy with which (a) the materials aredispensed and (b) the speed of the vehicle can be determined.

The speed of farm tractors and other off-highway equipment is not easilydetermined with accuracy. The conventional method of measuring the speedof a vehicle, i.e., by measuring the rate of revolution of the wheelswhich drive the vehicle, is not accurate when applied to farm tractors,for example, due to the high rate of slip of the driven wheels relativeto the ground. Measuring the speed of a tractor by measuring the rate ofrotation of the tractor's undriven wheels is also inaccurate because thewheels tend to skid during turning and to lift off the ground undercertain circumstances.

It has been recognized that the speed of land vehicles may be measuredusing doppler radar equipment. Doppler radar operates by broadcasting aradio frequency (RF) electromagnetic wave in a thin beam, and measuringthe frequency of the wave reflected from the ground relative to thefrequency of the broadcast wave. The difference between the twofrequencies is directly proportional to the speed of the vehicle.

The doppler radar apparatus should ideally produce a narrow radar beamwith substantially no side lobes, so that the beam can be pointed at adefined area of the ground and will not strike and be reflected fromadjacent structures, such as vehicle tires. The radiation pattern of theelectromagnetic wave generated by the doppler radar apparatus isdependent upon the characteristics of the antenna used with theapparatus. One type of antenna known to have low levels of side lobes isthe so-called "dual mode" horn antenna. Dual mode horn antennas aredescribed in the P. D. Potter article entitled "A New Horn Antenna WithSuppressed Side Lobe and Equal Beam Widths", the microwave journal,pages 71-78 (June, 1963).

Dual mode horn antennas are designed so that the electromagnetic wavepropagates through the horn in two modes. The radiation pattern of theantenna is a composite of the patterns produced by the two modes, andincludes substantially no side lobes since the side lobes produced byone mode cancel the side lobes produced by the other mode. The compositepattern thus produced has essentially no radiation energy outside of themain or axial lobe. In the past, it has been presumed that in order toproduce the proper boundary conditions for the two modes at the mouth ofa dual mode horn, the horn had to be designed to have a rather smallflare angle, on the order of 12.5°. A small flare angle results in arelatively long horn, however, since the length of a horn is establishedby its flare angle. Specifically, the diameter of the horn mouth isessentially determined in accordance with the desired gain and beamwidth of the resulting RF pattern. Given the preferred horn diameter offour to five inches for land vehicle applications using frequencies inthe 24 GHz range, a horn must be nearly two feet long in order to have a12.5° flare angle. A two foot long horn is simply too large to be ofpractical use on farm tractors and other off-road vehicles.

Even if a horn having the required radiation pattern and physical sizerequirements could be designed, problems would still be encountered inmaking the doppler radar system mechanically durable and reliable. Thehorn must be capable of withstanding the severe mechanical shocks towhich a farm tractor or other off-road vehicle will be subjected. Somematerials normally used in electromagnetic antennas or their componentsare not entirely suitable for use in off-road vehicles. The dielectriclenses sometimes used on horn antennas, for example, are generallyconstructed of pure polymeric materials. Such materials are brittle andsomewhat weak, and would therefore be subject to breakage if used on afarm tractor or other off-road vehicle. Other dielectrics can besubstituted for the polyethylenes normally used in RF lens structures.Any dielectric material that is used, however, must generally conform tocertain standards of homogeniety, since inhomogeneousness of thematerial may result in dispersion or polarization of the RF wave whichis being focused by the lens.

Furthermore, if more than one type of metal is used in the dopplerapparatus, differences in the responses of the metals to temperaturechanges can cause stressing or loosening of internal components of theapparatus. For example, the horn and its related microwave componentsare generally formed of aluminum, since aluminum is easily cast into thecomplex shapes in which the components are to be formed. The exteriorhousing and horn mounting components, on the other hand, shouldpreferably be formed of steel since steel is inexpensive and rugged.When an aluminum horn assembly is mounted within a steel structure,however, differences between the temperature coefficients of the twomaterials can cause stressing and loosening of joints betweencomponents, thereby degrading the durability or life of the apparatus.

Another problem of the typical doppler radar system relates to theobserved tendency of doppler radar systems to indicate that a vehicle ismoving when it is in fact stopped. One method of avoiding such erroneousindications would be to disable the speed indication provided by thedoppler radar whenever the vehicle tires remained stationary for morethan a selected time period. Such a solution to the problem complicatesthe electrical interconnection between the doppler radar system with therest of the vehicle, however, and is undesirable for that reason. In onedoppler radar system for a vehicle, the problem is solved by disablingthe speed indication whenever it is below a selected threshold speed. Itwould be desirable if a doppler radar system could be devised whichsimply did not provide the erroneous velocity indication in the firstplace.

SUMMARY OF THE INVENTION

The present invention relates to doppler radar velocity measurementapparatus that has electrical and mechanical characteristics whichrender it suitable for use on off-road vehicles such as farm tractorsand the like. The apparatus of the invention is a small, compact packagethat generates a narrow, well-defined beam of microwave energy havingsubstantially no side lobes. In addition, the apparatus providessubstantially zero output when the vehicle is halted, and is bothdurable and temperature resistant.

The doppler radar apparatus in accordance with the present inventionuses a dual mode horn antenna having a flare angle which issubstantially in excess of the 12.5° normally used. The horn used in thedescribed embodiment has a flare angle of 38.84°. Despite the wide flareangle, the horn does not suffer from the mode phasing problems which hadbeen anticipated. Moreover, because selection of the large flare angle,the length of the horn necessary to arrive at the desired horn mouthdiameter is substantially reduced. Thus, the dual mode horn is shortenough for use on off-road vehicles.

The horn employs a dielectric lens positioned over the horn so as toassist in focusing of the beam. The lens is formed of a glass filledpolymer. Surprisingly, the glass filling does not interfere with theelectrical characteristics of the lens. The glass filled polymerprovides a lens of substantially greater strength than conventionallenses formed, for example, of pure polyester material.

The doppler radar apparatus of the invention is designed such that thefit between the microwave horn assembly and the housing assembly doesnot loosen with temperature variations, even though the two assembliesare constructed of different materials. The housing assembly hasinterior surfaces which abut matching exterior surfaces of the microwavehorn assembly. The planes of the abutting surfaces are selected to beparallel to the direction of expansion or contraction of the assembliesdue to temperature variations. Unequal expansion or contraction of thedifferent structural materials of which the housing and horn assembliesare fabricated thus cause sliding of the abutting surfaces relative toone another rather than loosening of the fit between the assemblies.

It has been found that the tendency of a doppler velocity apparatus toprovide a nonzero velocity indication when the vehicle on which it ismounted is stationary is caused largely by vibrations imparted to thedoppler velocity apparatus by the idling vehicle engine. To solve theproblem, the apparatus of the invention includes vibration isolatorswhose spring rate and damping characteristics are selected in accordancewith the characteristics of a high-pass filter used to filter thedoppler signal provided by the apparatus. More particularly, the springrate and damping characteristics of the vibration isolator are selectedsuch that low level vibrations of frequencies which would producedoppler frequencies greater than the cut-off frequency of the high-passfilter are not transmitted to the microwave horn assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the present inventionwill become more readily apparent from the following detaileddescription, as taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a side elevation view of a farm tractor, indicating thelocation at which doppler radar apparatus could be mounted;

FIG. 2 is an exploded view of doppler radar apparatus in accordance withthe teachings of the present invention;

FIG. 3 is an exploded quarter section side view of the microwave hornassembly of the FIG. 2 apparatus;

FIG. 4 is a quarter section side view of the assembled unit;

FIG. 5 is a quarter section side view of the housing assembly of theFIG. 2 apparatus; and

FIG. 5A is an enlarged sectional view of the vibration isolators of thehousing assembly.

DETAILED DESCRIPTION

A doppler radar unit is largely self-contained and must be mounted on avehicle such that the beam of microwave energy which it generates willstrike and be reflected from the ground beneath the vehicle. The dopplerradar unit can be mounted either at the front or back of some vehicles,since in both places there is generally an unrestricted view of theground over which the vehicle is passing. On a farm tractor, however,front and rear mounted implements such as stackers and plows mayinterfere with the operation of the doppler radar. As a result, thedoppler radar unit must be mounted on the side of a tractor. The dopplerradar unit should preferably be mounted near the center of the tractor,and can be pointed either forward or rearward.

FIG. 1 shows a farm tractor 10 with a doppler radar unit 12 mounted at arepresentative location on the side of the vehicle. The beam 14generated by the unit illuminates the ground between the rear tires. Itis therefore important that the beam be confined in width. If the beamhas excessive spillover in the area of the wheels, microwave energy willbe reflected from the wheels which will substantially interfere with theoperation of the unit. To prevent this from occurring, the unit mustproduce a beam having a narrow width and virtually no side lobes. If thearea illuminated by the radar unit is too small, however, irregularitiesin the ground surface will produce marked deviations in the velocitymeasured by the system. By broadening out the beam to cover, forexample, 5°-8°, sufficient ground area is illuminated that a spatialaveraging effect takes place, whereby the mean frequency of the returnedmicrowave signal is accurately representative of the actual speed of thevehicle.

A doppler radar velocity measurement system in accordance with theteachings of the present invention is illustrated in partiallydisassembled form in FIG. 2. The system includes two major assemblies--amicrowave horn assembly 22 and a housing assembly 24. In addition, aprinted circuit board 26 is connected to the microwave horn assembly 22by an electrical cable 28.

A quarter sectional, side view of most of the major parts of themicrowave horn assembly 22 is shown in FIG. 3. For simplicity ofunderstanding, the elements are shown disassembled in FIG. 3. Themicrowave horn assembly 22 includes a microwave transceiver 32, awaveguide transformer 34, a mode generator and horn antenna 36, and ahorn cover 38 which incorporates a dielectric lens. Certain relatedelectronic circuits 39, 40, and 41, are contained on the printed circuitboard 26 (FIG. 2) which is mounted on the microwave horn assembly.

The microwave transceiver 32 is a conventional component and thereforewill not be described in detail. The transceiver 32 is not sectioned inFIG. 3. Generally, the transceiver includes a conventional GUNNoscillator for generating a linearly polarized microwave signal having afrequency of approximately 24.125 GHz. The oscillator is powered by apower supply 39, which is in turn connected to the vehicle battery. Thetransceiver 32 has a rectangular waveguide (not shown) formed in it forguiding the generated microwave signal to the waveguide transformer 34.The microwave signal propagates through the transformer 34, the horn 36,and the cover 38 into free space. The horn assembly 22 will preferablybe mounted such that the polarization vector of the transmitted signalis vertically oriented. The returned signal (reflected from the ground)is gathered by the horn 36, and travels through the transformer 34 backinto the transceiver 32. The transceiver mixes the returned signal withthe original signal, thereby forming a doppler signal the frequency ofwhich is equal to the difference between the frequency of the originalsignal and the frequency of the returned signal. The doppler signal iscoupled to an external terminal 35 on the transceiver.

The doppler signal output terminal 35 of the transceiver 32 is connectedto a high pass flter 40. The high pass filter eliminates all frequenciesbelow a breakpoint of around 50 Hz. As will be described furtherhereinafter, the filter 40 has electrical characteristics whichcomplement the mechanical characteristics of two mechanical vibrationisolators incorporated in the housing assembly 24. The filter andvibration isolators cooperate to eliminate erroneous velocity readingswhen the vehicle is stopped. The output of the high pass filter isconnected to the input of a signal processing circuit 41. The circuit 41processes the high-pass filtered doppler signal and provides a processedsignal at an output terminal 42, which represents the output of thesystem. The signal provided on output terminal 42 has a frequency whichis directly proportional to the velocity of the vehicle on which thedoppler radar apparatus is mounted.

The waveguide transformer 34 is included in the horn assembly to providea smooth electromagnetic transition between the rectangular waveguide ofthe transceiver 32 and a circular waveguide section 46 which is locatedat the throat of the horn antenna 36. The transformer section 34includes a waveguide having three distinct rectangular cross sectionportions 43, 44 and 45. The widths of the three portions 43, 44 and 45(i.e., the dimension measured in a direction normal to the plane of thepaper, as viewed in FIG. 3) are all equal. The heights of the threesections are, however, different. The height of the first portion 43 isless than its width, but both the height and width match thecorresponding dimensions of the waveguide section of the transceiver 32.Thus, when the transceiver 32 is bolted to the waveguide transformer 34(by bolts not shown), the two waveguides are aligned with one anotherand provide a single waveguide of uniform cross sectional shape. Theheight of the third transformer portion 45 matches its width so that theportion has a square cross section. The height of the intermediateportion 44 is intermediate the heights of the other two portions 43 and45.

The horn antenna 36 includes a circular waveguide 46 which is coaxialwith the axis of symmetry A of the three transformer portions 43, 44,and 45 when the horn and the transformer are bolted together. Thediameter of the circular waveguide 46 is slightly greater than theheight of the square cross sectional portion 45 of transformer 34, andis such that the 24.125 GHz RF wave predominantly propagates through thewaveguide 46 in the circular TE₁₁ mode. A second mode is, however,excited by a step transition 48 in the diameter of the cylindricalwaveguide 46. (The excitation of a second mode in this manner is known,and is described in the previously mentioned Potter article.) The RFsignal passing through the flared section 50 of the horn antenna 36therefore includes two modes, the predominant TE₁₁ mode and the higherorder TM₁₁ mode. Both modes propagate through the flared section 50 ofthe horn, and pass from there into free space. The horn 36 is apyramidal conical horn. The axis A coincides with the axis ofcylindrical symmetry of the horn 50.

The horn antenna 36 produces a radiation pattern having a center (knownin the art as the boresight of the antenna) which is coincident with theaxis of symmetry A of the horn 36. The radiation pattern of the hornantenna 36 is a composite of the radiation patterns produced by the TE₁₁and TM₁₁ waves. The pattern produced by the TE₁₁ wave has a pronouncedaxial peak, and off-axis side lobes of various amplitudes. The TM₁₁mode, on the other hand, produces a pattern which lacks an axial peak.The TM₁₁ pattern includes off-axis side lobes which effectively cancelthe side lobes of the pattern produced by the TE₁₁ mode. The resultingcomposite radiation pattern therefore consists essentially of thepronounced central peak alone, with very little energy dispersed in theside lobes.

The gain of the horn antenna 36 and the width of the main lobe of itsradiation pattern are both related to the mouth diameter of the horn 36.For use of the doppler unit 12 on a farm tractor, a preferred pattern ofground illumination was produced by a beam width on the order of 8°. Thebeam width, coupled with the frequency being used (24.125 GHz), dictatedthat the horn mouth should be on the order of four to five inches. Ithas been conventional wisdom that, in order for the TE₁₁ and TM₁₁ wavesto be phased properly at the horn mouth, the slope of the flared orconical section of the horn should be on the order of 6.25° with respectto axis A, giving a flare angle of 12.5°. For a 12.5° flared horn tohave a four inch diameter mouth, however, the flared section must beabout two feet long. A two foot horn is too long to be mountedconveniently on the side of a tractor or most other vehicles.

It has now been found that the flare angle on the conical section of thehorn antenna may be increased by as much as two or three times withoutadversely affecting the phasing of the TE₁₁ and TM₁₁ waves. In thespecific example being described, the flare angle of the conical section50 is about 39°. This flare angle, coupled with a mouth diameter ofapproximately 4.35 inches, results in a horn length of only about 5.27inches.

A horn antenna with a large flare angle does tend to disperse the beamof microwave energy passing through it. The dispersive effects of theincreased flare angle are mitigated by including a dielectric lenselement in the horn cover 38. The horn cover 38 shown in FIG. 3 includesa dielectric lens 60 and a cylindrical rim 62. The lens 60 is circularlysymmetrical about the axis A and has a planar outer surface 64. Theinner surface of the lens 60 is formed by plural concentric annularsteps 66, the axial thickness of the lens being constant across thewidth of each step. The contour of the inner surface of the lens, whichis established by the differences in the thickness of the lens at thevarious steps 66, is convex and is selected so that the focal point ofthe lens substantially coincides with the phase center of the antenna.The antenna phase center is located within the throat of the horn.

The cylindrical rim 62 of the horn 38 is formed in one piece with thelens 60, and joins the outer perimeter of the lens. The rim 62 has ashort tubular portion with plural axially projecting tabs 68circumferentially spaced around the end of the rim 62 opposite the lens60. The tabs 68 are formed with radially directed beads 70 that engagean annular ridge 72 cast into the outer perimeter of the horn 36. Whenthe horn cover 38 is pushed over the end of the horn 36, the tabs 68flex outward over the ridge 72, and then snap back in place so that thebeads engage the annular ridge and hold the horn cover 38 firmly inplace over the end of the horn antenna 36. A tight, environmentallysecure seal between the horn cover 38 and the horn antenna 36 is assuredthrough use of an O-ring 78. The O-ring is carried in an annular groove80 in the exterior surface of the horn antenna 36 between the ridge 72and the outer edge 76 of the horn.

When the horn cover is in place, the outer edge 76 of the horn 36 abutsan annular lip 74 which projects radially inward from the cylindricalrim 62. The portion of the rim between the annular lip 74 and the pointat which the rim joins the lens 60 functions as a axial spacer, holdingthe lens 60 at a predetermined axial distance from the horn antenna 36.The axial spacing of the lens 60 established by the annular lip 74 isselected to minimize the voltage standing wave ratio (VSWR) within thehorn antenna 36 caused by reflections from the dielectric lens 60.

The lens 60 must endure significant mechanical stresses during thelifetime of the doppler radar apparatus, and hence must be constructedof a strong and durable material. Normally such lenses are formed of ahomogeneous polymeric material, such as polyethylene. Normal homogeneouspolymeric materials have relatively low durability and shock resistance,however. It is known that glass filled polymers have greater strengthand durability than pure polymers. Nonetheless, in the past it has beenpresumed that glass filled polymers could not be employed to focusfrequencies in the gigahertz range since the glass particles would causethe polymer to appear inhomogeneous to those frequencies, resulting indispersion and possibly polarization of the RF beam. Uponexperimentation, however, it has been found that a dielectric lensformed of a glass filled polymeric material does not cause the expectedRF dispersion and/or polarization. It is therefore both possible anddesirable to form the dielectric lens of a glass filled polymericmaterial. In the specific embodiment being described, the lens is formedby injection molding of a 40% glass filled polyester sold by RTP Companyof Winona, MN, under the trade name "Fiberite No. 1007". This materialis not only strong and durable, but also (unlike pure polyester) has acoefficient of thermal expansion which closely matches the coefficientof thermal expansion of the aluminum horn. The horn 36 and horn cover 38thus expand and contract essentially as one unit in response to localtemperature changes.

In addition to the elements described above, the microwave horn assembly22 includes a mounting adapter 90 and a locking ring 108 (shown in FIGS.2 and 4). The mounting adapter 90 is a one-piece, cast aluminium memberthat includes two axially spaced annular rings 92 and 94 joined byspacer portions 96 and 98. The spacer portions 96 and 98 extend axiallybetween the perimeters of the two annular rings 92 and 94 atdiametrically opposed circumferential positions. The central opening inannular portion 94 has a large enough diameter that the adapter 90 canreceive both the microwave transceiver 32 and most of the waveguidetransformer 34. At its end adjacent the horn antenna 36, the waveguidetransformer has a mounting flange 100 with an outer diameterapproximately the same as the outer diameter of annular portion 94 ofadapter 90. The annular portion 94 of adapter 90 thus abuts the flange100, which in turn abuts a similar flange 102 carried by the horn 36.The three elements, mounting adapter 90, waveguide transformer 34 (withtransceiver assembly 32 attached), and horn antenna 36 are firmlyattached to one another by lug bolts. One of the lug bolts 103 is shownin FIG. 4. The lug bolts project through axial holes in the annularportion 94 of adapter 90 and the mounting flange 100 and are screwinginto threaded holes (not shown) in the mounting flange 102.

The second annular portion 92 of the mounting adapter 90 includes acylindrical outer wall 104 having threads 106 formed on its outersurface for receiving the locking ring 108 (see FIGS. 2 and 4). Themicrowave horn assembly 22 is held in place within the housing assembly24 by the locking ring 108. Moreover, as will be brought outhereinafter, the locking ring 108 also holds the elements of the housingassembly 24 together.

FIG. 4 shows how the microwave horn assembly 22 is mounted within thehousing assembly 24. For simplicity of description, the housing assembly24 is illustrated separately in FIG. 5. As is shown in FIG. 5, thehousing assembly 24 includes a generally cylindrical outer housing 200,a generally cylindrical inner housing 202, two identical toroidalvibration isolators 204 and 206, an end cover 208, a splash shield 210and a bracket 211 (shown in FIG. 2) for attaching the housing to atractor. The microwave horn assembly 22 seats against interior annularsurfaces of the two vibration isolators 204 and 206, and is isolatedfrom vibrations applied to the outer housing 200 due to the axial andradial resiliency of the vibration isolators. The vibration isolators204 and 206 are identical, but face opposite axial directions. Eachincludes inner and outer steel rings 212 and 214 and a resilientintermediary material 260. The intermediary material 260 is cast inplace or injection molded between the two rings 212 and 214. In theembodiment of the invention being described, the intermediary material260 is a silicon elastomer (type 3FC510B37C20E016G11EA14, according toclassification conventions established by publication D2000 of theAmerican Society of Testing and Materials). The silicon has a hardnessof about 30-40 durometer.

The isolation characteristics of the vibration isolators 204 and 206 areestablished, in part, by the shape of the resilient material 260sandwiched between the annular rings 212 and 214. As can be seen moreclearly in FIG. 5A, the resilient material 260 includes an outer annularportion 218 in which the outer ring 212 is embedded, and an innerannular portion 220 bonded onto the radially outer surface of the innerring 214. The two annular portions 218 and 220 are joined by a thin,conical portion 222. The thickness and shape of the conical portion 222of the resilient material 260 cause vibrational forces transmitted fromthe outer housing 200 to be absorbed initially and primarily bydeflection of the portion 222. Because the portion 222 is disposed at anacute angle to the axis of symmetry A of the unit 12, the portion 222 iseffective to absorb both radial and axial vibrations and gives eachisolator 204 and 206 essentially the same spring rate in respons to bothradial and axial vibrations. If a shock load is applied to an isolatoror if the vibrational forces become so great that the conical portion222 collapses and permits the annular portions 218 and 220 to abut eachother, the spring rate of the isolator will increase substantially dueto the compression loading of the resilient material.

The initial spring rate and damping characteristics of the vibrationisolators 204 and 206 are established by the thickness T of the conicalportion 222 and by the material of which it is formed. In accordancewith one aspect of the present invention, the spring rate and dampingcharacteristics of the vibration isolator are selected in accordancewith the cutoff frequency of the high pass filter 40 which filters theoutput of the microwave transceiver 32. As has been describedpreviously, the cutoff frequency of the high pass filter (FIG. 3) isabout 50 Hz. Consequently, any vibrational motion which produces adoppler frequency below the frequency of 50 Hz will not be perceptibleto the circuitry which responds to the output of the low pass filter.Thus, it is desirable that the material and thickness T of the conicalportion 222 of the resilient material 260 be selected so that themicrowave horn assembly is isolated from low level vibrationsfrequencies which would produce doppler frequencies in excess of 50 Hz.

The preferred thickness T was determined largely empirically. Aparticular thickness T was selected and then the resonant frequency ofthe system was determined by shaking the outer housing at differentfrequencies and measuring the accelerations experienced by the suspendedcomponents (i.e., the horn assembly 22, inner housing 202, and innerportions of the vibration isolators). The resonant frequency (f) of thesystem is related to the mass (m) of the suspended components (whichweigh about 3.5 lbs in the example being described) and the spring rate(k) of the vibration isolators by the equation f=(k/m)^(1/2) /(2). Itwas determined empirically that the resonant frequency (f) had to bebelow 40 cycles per second in order to isolate the horn assembly fromvibrations which would produce doppler signal frequencies above thecutoff frequency of the high pass filter 40 (FIG. 3).

With the particular silicon elastomer used in the embodiment of theinvention being described, it was found that the thickness T had to beapproximately 0.030 inches in order for the initial spring rate of thevibration isolator to provide the desired isolation. A thickness of0.030 inches, however, was not sufficient in itself to lower theresonant frequency of the vibration isolators below the point at whichdoppler frequencies in excess of 50 Hz were produced. The spring rateand thus the resonant frequency of the vibration isolators was reducedstill further by slightly buckling the conical portion of the resilientmaterial 260. The conical section 222 was buckled by axially displacingthe inner annular ring 214 of each isolator 204 and 206 as compared tothe positions of the rings with the resilient material 260 unstressed.The relative displacement of the two rings is achieved by appropriateselection of the length of the interior housing 202, which establishesthe axial spacing between the inner axial rings 214 of the two vibrationisolators 204 and 206.

More specifically, the interior housing 202 extends between a radiallydirected lip 228 on the interior ring 214 of vibration isolator 204 andthe corresponding lip of the interior ring of vibration isolator 206.The diameter of the inner housing matches the mean diameter of the lip228, whereby the tube and each ring 214 abut throughout theircircumferences. The inner housing 202 is held in the proper transverselocation relative to the lips 228 by rims 229. The rims 229 extendaxially inward toward the center of the assembly 24 and have diameterssomewhat greater than the diameter of the spacer tube. The ends of innerhousing 202 are received within the rims, and are thus held in coaxialalignment with the lips 228.

The axial locations of the exterior rings 212 of the vibration isolators204 and 206 are established by two radially inwardly projecting annularridges 230 and 232 in the outer housing 200 of the housing assembly 24.Each of the vibration isolators 204 and 206 is inserted axially into acorresponding end of the outer housing 200 until it abuts the adjacentannular ridge 230 or 232. The resilient material between the exteriorring 212 of each vibration isolator and the housing 200 is compressedupon insertion of the isolator into the housing, thus producing a tightfriction fit. The friction fit prevents the vibration isolators frommoving away from the corresponding ridges 230 and 232.

The spacing between the two annular ridges 230 and 232 is less than thelength of the inner housing 202. Upon assembly of the isolators 204 and206 with the inner and outer housings 202 and 200, therefore, theinterior ring 214 of each vibration isolator is axially displaced fromits normal position relative to the corresponding outer ring 212. Thisbuckles the conical portion 222 of the resilient material 260, asdescribed previously, and reduces the spring rate of the conical portion222. The suspension then has a first resonant frequency of about 30cycles per sec, which is low enough that vibrations which would producedoppler frequencies in excess of 50 Hz are isolated from the microwaveassembly.

As can best be seen in FIG. 4, the microwave horn assembly 22 is carriedwithin the housing assembly 24 by two frustoconical surfaces 234 on theinterior annular rings 214 of the vibration isolators 204 and 206. Eachof the frustoconical surfaces 234 is symmetrical about the axis A and isflared radially outwardly in an axial direction away from the center ofthe housing assembly 24. A corresponding frustoconical surface of themicrowave horn assembly 22 abuts each frustoconical surface of anannular ring 214. One of the frustoconical surfaces of the microwavehorn assembly 22 is frustoconical surface 236 (best seen in FIG. 3) onthe horn 36. The frustoconical surface 236 includes a circumferentialgroove that receives an O-ring 238 to provide a secure environmentalseal between the vibration isolator and the horn assembly. The otherfrustoconical surface of the microwave horn assembly 22 is afrustoconical surface 240 (FIG. 4) on the locking ring 108. There is noO-ring on the locking ring 108 because the rear end of the assembly isprotected by a gasket 242 between the housing cover 208 and the outerhousing 200.

The microwave horn assembly is locked into position against thevibration isolators 204 and 206 by the locking ring 108. The lockingring 108 is screwed onto the mounting adapter 90 after the microwavehorn assembly has been inserted into the housing assembly 24 through itslefthand end (as viewed in FIG. 4). The single locking ring 108 thusholds the microwave horn assembly firmly within the housing assembly 24.Since the locking ring also applies axial pressure against the two innerrings 214 of the vibration isolators 204 and 206, it also holds thevibration isolators 204 and 206 against the ends of the inner housing202.

The major structural elements of the microwave horn assembly 22,including the transceiver assembly 32, waveguide transformer 34, horn36, and mounting adapter 90 are all cast aluminum. The elements of thehousing 24 shown in FIG. 5, on the other hand, including the inner andouter tubes 200 and 202 and the outer and inner rings 212 and 214 of thevibration isolators 204 and 206, are all fabricated of steel. Sincesteel and aluminum have different coefficients of thermal expansion,changes in temperature would normally tend to cause loosening of the fitbetween the aluminum and steel elements. The possibility of suchloosening is avoided in the apparatus 12 by carefully selecting theangles of the abutting frustoconical surfaces 234, 236 and 240 relativeto the axis A.

More specifically, the abutting surfaces of vibration isolators 204 and206 and microwave horn assembly 22 lie on two imaginary conical surfaceshaving a common apex "C" located on the axis A midway between the twovibration isolators 204 and 206. Point C represents the center ofexpansion of both the aluminum horn assembly and the steel housingassembly. Thus, in response to thermal expansion or contraction of theassemblies, any given point on either assembly will move along a linepassing through both that point and the central point C. The contactsurfaces between the assemblies are aligned at an angle A relative tothe axis A such that they can be extended to pass through the point C.The directions of expansion and contraction of the abutting aluminum andsteel surfaces are therefore parallel to one another. As a result, thesurfaces will slide across one another rather than separating.

To take full advantage of the thermal stability afforded by thearrangement described above, the locking ring 108 should be fabricatedof the same material as the horn assembly, i.e., of aluminum. Analuminum locking ring is somewhat difficult to remove once it has beeninstalled, however. It may in some circumstances be desirable to avoidthis by fabricating the locking ring of some material other thanaluminum, such as brass.

Once the microwave horn assembly has been locked in place within thehousing assembly 24 by the locking ring 108, the housing cover 208 isbolted onto a mounting flange 244 formed at one end of the outer housing200 of the housing assembly 24. The splash shield 210 is glued onto theother end of the outer tube 200.

Apparatus has thus been described for use in a doppler velocitymeasuring system for use on farm tractors and the like. The apparatusincludes a novel dual mode horn, including a flare angle substantiallyin excess of 12.5°, and a dielectric lens formed of a glass filledpolymer. The doppler signal provided by the RF transceiver associatedwith the dual mode horn is high pass filtered to remove low frequencysignals from the signal. The horn is mounted in such a way thatmechanical vibrations which would induce doppler signal frequencies inexcess of the cut-off frequency of the high pass filter are nottransmitted to the horn assembly. Moreover, the aluminum horn assemblyand the steel housing assembly are coupled together in such a way thatdimensional changes of the materials due to temperature changes will notproduce a loosening of the friction fit between the component parts ofthe system. Furthermore, the entire system is easily assembled, and islargely held together by a single locking ring.

Although the invention has been described with respect to a preferredembodiment, it will be appreciated that various rearrangements andalterations of parts may be made without departing from the spirit andscope of the present invention, as defined in the appended claims. Forexample, it is possible to eliminate the inner housing 202 altogether.Tightening the locking ring onto the mounting adapter then causesstretching rather than buckling of the vibration isolators. It has beenfound that vibration isolators in this tension mode have characteristicssimilar to those of vibration isolators in a buckling mode. Thedisadvantage of this approach is that the parts cannot be fastened astightly as when the inner housing is included. Other variations of thedescribed apparatus will be apparent to those skilled in the art.

What is claimed is:
 1. Apparatus for use in a doppler radar velocitymeasurement system, comprisingmeans including a rectangular waveguidefor generating an RF wave in said rectangular waveguide, said RF wavehaving a frequency in the range of 24 GHz, waveguide transformer meansfor providing a smooth RF transition between said rectangular waveguideand a circular waveguide, a circular pyramidal horn antenna having aflared section with a mouth and a throat, said horn also including acircular waveguide section formed at the throat of said flared section,said circular waveguide being coupled to said waveguide transformer forreceiving said RF wave from said waveguide transformer and beingdimensioned so that said RF wave can propogate in only the TE₁₁ mode,mode generator means disposed at the junction of said circular waveguideand the throat of said flared section for generating a TM₁₁ mode RFwave, said flared section guiding both RF wave modes from said modegenerator means into free space, wherein said flared section has aninterior mouth diameter of about 4.4 inches and a flare angle of about39° so that the length of said flared section is only about 5.3 inches,and a dielectric lens positioned across the mouth of said flaredsection, said RF lens being spaced axially from the mouth of said hornso as to reduce the standing wave ratio in said horn, and having a focallength corresponding approximately to its axial spacing from the phasecenter of said horn antenna.
 2. Apparatus for use in a doppler radarvelocity measurement system, comprisinga microwave horn assembly forreceiving and transmitting an RF electromagnetic wave, said assemblyproviding an electrical doppler signal having a frequency equal to thedifference between the frequencies of said transmitted and receivedwaves, electronic filter means responsive to said doppler signalprovided by said microwave horn assembly for high pass filtering saidsignal to provide an output signal that includes only frequencies abovea predetermined cutoff frequency, and means for resiliently mountingsaid microwave horn assembly on a vehicle, said resilient mounting meanshaving spring rate and damping characteristics such that said microwavehorn assembly is isolated from low level vibrations having frequencieswhich would produce doppler signals with frequencies greater than thecutoff frequency of the high pass filter means, whereby erroneousdoppler signals produced by horn vibrations are effectively eliminated.3. Apparatus as set forth in claim 2, wherein said resilient mountingmeans includes (a) at least two annular vibration isolators encirclingand firmly engaging said microwave horn assembly at two axially spacedlocations along said assembly, and (b) rigid means engaging saidvibration isolators for attaching the isolators to said vehicle. 4.Apparatus as set forth in claim 3, wherein said resilient mounting meansfurther includes a spacer tube arranged between and abutting with saidvibration isolators, and wherein said horn assembly includes (a) atleast two discrete components, each having a radially extending surfaceadapted to abut a corresponding one of two circumferential surfaces onopposing axial ends of said resilient mounting means and spacer tubearrangement, and (b) means for fastening said first and second partsaxially together when assembled with said resilient mounting means andspacer tube arrangement.
 5. Apparatus as set forth in claim 4, whereinsaid fastening means comprises complementary inner and outercircumferential threads on said two discrete components, said threadspermitting said components to be screwed axially together.
 6. Apparatusas set forth in claim 3, wherein each vibration isolator includes a thinfrustoconical annulus of resilient material, first means coupled to theradially inner perimeter of said frustoconical annulus for providing asurface for engaging said horn assembly, and second means coupled to theradially outer perimeter of said frustoconical annulus for providing asurface for engaging said rigid mounting means.
 7. Apparatus as setforth in claim 6, wherein said first and second surface providing meanscomprise first and second rigid rings to which said resilient materialis bonded.
 8. Apparatus as set forth in claim 7, wherein saidfrustoconical annuli of resilient material in the two vibrationisolators are oriented in opposite directions, and wherein saidresilient mounting means further comprises means for axially spacingsaid two first concentric rings from one another by a first amount andsaid two second concentric rings from one another by a second amount,said first and second amounts being such that each said frustoconicalannulus is slightly buckled.
 9. Apparatus as set forth in claim 7,wherein the first and second rigid rings of each of said vibrationisolators have radially facing portions which are formed of resilientmaterial and which are disposed at overlapping axial locations such thatsudden mechanical shocks causing momentary collapse of saidfrustoconical annuli are absorbed by the resilient impact of saidresilient facing portions of said first and second rings.
 10. Apparatusas set forth in claim 9 wherein each said ring is composed of aresilient material but has a rigid ring adjoined thereto to providedimensional stability.
 11. Apparatus for use in a doppler velocitymeasurement system, comprisinga microwave horn assembly for transmittingand receiving an RF electromagnetic wave and providing a doppler signalhaving a frequency related to the velocity of the vehicle to which thesystem is attached, and a housing assembly for housing said hornassembly, said housing assembly being formed of a structural materialhaving a coefficient of thermal expansion which is different than thecoefficient of thermal expansion of the material of which said hornassembly is formed, said housing assembly having interior surfaces whichabut matching exterior surfaces of said microwave horn assembly, whereinthe planes of said surfaces are all selected to be parallel to thedirection of movement of the material underlying said surfaces due tothermal expansion or contraction, whereby unequal thermal expansion orcontraction of said materials of which said assemblies are formed causessaid abutting surfaces to move parallel to one another rather thannormal to one another.
 12. Apparatus as set forth in claim 11 whereinsaid microwave horn assembly is aluminum and said housing assembly issteel.
 13. Apparatus as set forth in claim 11, wherein said microwavehorn assembly is generally cylindrical and has two frustoconicalsurfaces coaxial with said generally cylindrical horn assembly, whereinsaid surfaces have equal and opposite degrees of obliquity relative tosaid axis such that, if said surfaces were extended to form cones, thecones would be similar and would have a common apex disposed betweenthem along the axis, and wherein said housing assembly has matchingsurfaces which abut said surfaces of said horn assembly.
 14. Apparatusas set forth in claim 13, wherein each of said frustoconical surfaces ison a different part of said horn assembly, wherein said parts areadapted to be assembled with said housing assembly by being insertedaxially through different ends of said housing assembly until saidfrustoconical surfaces of said parts meet the corresponding surfaces ofsaid housing assembly, and wherein means are included for fastening saidtwo parts together in the assembled position.
 15. Apparatus as set forthin claim 14, wherein said fastening means draws said parts togetheraxially, thereby insuring a tight friction fit between said surfaces ofsaid assemblies.
 16. Apparatus as set forth in claim 13 wherein said twofrustoconical surfaces are on respective first and second parts of saidmicrowave horn assembly, and where one of said parts carries coaxialouter threads and the other of said parts carries matching coaxial innerthreads, whereby said horn assembly and housing assembly are assemblableby inserting said first and second parts axially through different endsof said housing assembly and screwing said two parts axially togetheruntil a tight friction fit is formed between said matching frustoconica1surfaces of said housing assembly and said first and second parts. 17.Apparatus for use in a doppler radar velocity measurement system,comprising:means for generating a dual mode RF electromagnetic wave andfor guiding said wave to a horn; a conical horn having a small diameterend and a large diameter end and being coupled adjacent its smallerdiameter end to said generating means for transmitting said dual mode RFwave into free space, said horn having a flare angle substnatially inexcess of 12.5°; lens means covering the large diameter end of saidhorn, said lens means having a focal length substantially equal to thedistance from the phase center of said horn to said lens; saidgenerating means including microwave transceiver means for generatingsaid RF wave and for receiving said RF wave back again after reflectionfrom an obstacle, said transceiver means providing a doppler signalhaving a frequency equal to the frequncey difference between saidgenerated and received RF waves; high pass filter means for filteringsaid doppler signal to eliminate all frequencies in said doppler signalbelow a selected cutoff frequency; and resilient means for mounting saidgenerating means, horn and lens means on a vehicle, said resilient meanshaving characteristics selected such that said generating means isisolated from low level vibrations having frequencies which wouldproduce doppler signal frequencies in excess of the cutoff frequency ofsaid high pass filter means.
 18. Apparatus as set forth in claim 17,wherein said flare angle is in excess of 25°.
 19. Apparatus as set forthin claim 17, wherein said lens is formed of a glass filled polymer. 20.Apparatus as set forth in claim 17, wherein said lens is formed of 40%glass filled polyester.
 21. Apparatus as set forth in claim 17, whereinsaid generating means provides an RF wave having a frequency in therange of 24 GHz, and wherein said horn has length of about 5.3 inchesand an interior diameter at its large diameter end of about 4.4 inches.22. Apparatus for use in a doppler radar velocity measurement system,comprising:means for generating a dual mode RF electromagnetic wave andfor guiding said wave to a horn; a conical horn having a small diameterend and a large diameter end and being coupled adjacent its smallerdiameter end to said generating means for transmitting said dual mode RFwave into free space, said horn having a flare angle substantially inexcess of 12.5°, said horn and generating means are part of a microwavehorn assembly; lens means covering the large diameter end of said horn,said lens means having a focal length substantially equal to thedistance from the phase center of said horn to said lens; and a housingfor said horn assembly, said housing being fabricated of a materialhaving a coefficient of thermal expansion different from the coefficientof thermal expansion of a material of which said horn assembly isformed, said housing having interior surfaces which abut correspondingexterior surfaces of said horn assembly, said abutting interior andexterior surfaces being aligned at angles to a central longitudinal axisof the horn assembly selected so that expansion and contraction of thematerials caused by temperature variation will produce movement of saidabutting surfaces parallel to one another rather than normal to oneanother.
 23. Apparatus as set forth in claim 22, wherein said flareangle is in excess of 25°.
 24. Apparatus as set forth in claim 22,wherein said lens is formed of a glass filled polymer.
 25. Apparatus asset forth in claim 22, wherein said lens is formed of 40% glass filledpolyester.
 26. Apparatus as set forth in claim 22, wherein saidgenerating means provides an RF wave having a frequency in the range of24 GHz, and wherein said horn has length of about 5.3 inches and aninterior diameter at its large diameter end of about 4.4 inches. 27.Apparatus for use in a doppler radar velocity measurement system,comprising:means for generating an RF electromagnetic wave having afrequency of at least 24 GHz and for guiding said wave to a horn; a horncoupled to said generating means for transmitting said RF wave into freespace; a dielectric lens covering the mouth of said horn, said lensbeing formed of a glass filled polymer material; and said means forgenerating an RF electromagnetic wave comprises means for generating awave in the dominant TE₁₁ mode and wherein said horn includes meansresponsive to said TE₁₁ mode RF wave for producing a TM₁₁ mode RF wavetherefrom, whereby the generated RF wave propagates through said horn inboth the dominant TE₁₁ and the higher order TM₁₁ modes.
 28. Apparatusfor use in a doppler radar velocity measurement system, comprising:meansfor generating an RF electromagnetic wave and for guiding said wave to ahorn; a horn coupled to said generating means for transmitting said RFwave into free space; a dielectric lens covering the mouth of said horn,said lens being formed of a glass filled polymer material; saidgenerating means comprises microwave transceiver means for generatingsaid RF wave and for receiving said RF wave back again after reflectionfrom an obstacle, said transceiver means providing a doppler signalhaving a frequency equal to the frequency difference between saidgenerated and received RF waves; high pass filter means for filteringsaid doppler signal to eliminate all frequencies therein below aselected cutoff frequency; and resilient means for mounting saidgenerating means, horn and lens means on a vehicle, said resilient meanshaving characteristics selected such that said generating means isisolated from low level vibrations having frequencies which wouldproduce doppler signal frequencies in excess of the cutoff frequency ofsaid high pass filter means.
 29. Apparatus for use in a doppler radarvelocity meansurement system, comprising:means for generating an RFelectromagnetic wave and for guiding said wave to a horn; a horn coupledto said generating means for transmitting said RF wave into free space,said horn and said generating means are part of a microwave hornassembly; a dielectric lens covering the mouth of said horn, said lensbeing formed of a glass filled polymer material; and a housing for saidmicrowave horn assembly, said housing being fabricated of a materialhaving a coefficient of thermal expansion different from the coefficientof thermal expansion of a material of which said microwave horn assemblyis formed, said housing having interior surfaces which abut matchingexterior surfaces of said horn assembly, said abutting interior andexterior surfaces being aligned at an angle to a central longitudinalaxis of the horn assembly selected so that expansion and contraction ofthe materials which is caused by temperature variations will producemovement of said abutting surfaces parallel to one another rather thannormal to one another.